The disclosure concerns a plasma reactor used in fabrication of microelectronic semiconductor circuits. Such a reactor includes a vacuum chamber for processing a workpiece, such as a semiconductor wafer, an RF power applicator and process gas injection apparatus. Control of plasma ion density distribution within the chamber is essential in order to ensure uniformity of processing, or a uniform distribution of etch (or deposition) rate across the surface of the workpiece. The vacuum chamber is typically configured as a cylinder so as to optimize uniformity of plasma ion distribution in a radial direction and about an azimuthal direction. As employed herein, the term azimuthal direction refers to a rotational direction around the axis of symmetry of the cylindrical chamber.
One problem is that the vacuum chamber itself includes non-symmetrical features that interrupt the cylindrical symmetry of the chamber and therefore create non-uniformities in plasma ion distribution. This may be due, for example, to the effect such asymmetrical features have on the electromagnetic environment of the plasma or on the gas flow distribution within the chamber, or both. Such non-symmetrical features may include a vacuum port in the floor of the chamber and a workpiece (wafer) slit port (“slit valve”) through which the wafer is inserted into and removed from the chamber. These features tend to produce non-uniformities in plasma ion distribution in the azimuthal direction, or azimuthal non-uniformities.
That is needed is a way of reducing or eliminating such azimuthal non-uniformities in plasma ion distribution.